The design of railway hopper cars is governed by three main requirements. First, the fully loaded weight of the car must not exceed 286,000 lb. Thus to maximize useful, load car designers try to minimize car weight. At present an empty grain hopper car may typically weigh about 70,000 lb., such that lading in excess of 200,000 lb. is permissible. Second, the car must withstand a draft load which may be in excess of 500,000 Lbs. Third, the car must not buckle under buff loads when slowing or stopping. Under the first, dead weight, loading condition the car may be modelled as a simply supported hollow beam carrying a distributed vertical load in excess of 200,000 lb., with a corresponding bending moment distribution. Under the second, tensile draft, and third, compressive buff, loading conditions the car is like a column, taking tensile and compressive loads.
The general structure of contemporary curved-sided hopper cars can be idealized as a load bearing monocoque in the form of a hollow, downwardly opening, generally C-shaped, thin walled, low aspect ratio column. At each column end, the load is transferred through a transition structure from the shell into a stub sill and coupler by which the railcar is connected to the next rail car. The challenge in designing the structure for a hopper car, in general, is to reduce the mass of the thin shell, and any supporting structure, to a minimum while still maintaining the structural integrity required to withstand the given loads, and to transfer those loads between the couplers and the body shell. When the shell is made too thin it fails in compression due either to global buckling of the structure, or to the local buckling phenomenon of wrinkling. In such a hollow shell structure, the ability to resist the compressive buff load, without buckling, requires that the principle longitudinal structural components of the car, those being the roof and side walls, work together as a single integrated structure.
The hopper car's side structure contributes to its ability to withstand compressive buff loads and lateral loads in comers as well as the customary loads experienced due to lading. The side structure and the roof structure also interact to stabilize each other. Side sheets have been made of several rolled sheets cut to the arc length measured from the side sill to the top chord, with their rolling direction perpendicular to the longitudinal axis of the car, butt welded together along their side edges. The side sheets require a significant amount of assembly time and effort, and the resulting butt-welded seams are oriented perpendicularly to cyclic tensile draft loads.
Hopper car designs also face the difficulty of arranging the transition structure for carrying loads from the end hoppers to the shear plates and bolsters which actually rest on the trucks, that is, in the area where the shear plate, the end hopper slope sheet, and the hopper come together. Current industry designs do not tend to increase the stiffness of the side construction from the bolster toward the end hopper compartment. It is advantageous to provide an increase in the local stiffness of the hopper shear plate, the hopper sheet extension, and the side sill, but without increasing the thickness of those members over their full lengths. If the side sills are made thicker over their entire lengths, a large amount of material would be added that would not be used effectively.
In general, it would be advantageous to have an improved hopper car shell structure. It would be advantageous to have, and there has been a long felt need for, an improved hopper car side sheet. Finally, there has been a long felt need for an improved structure to transfer the load from the hopper car shell structure to the trucks of the railcar.